The aims of this project are to identify the variables important for the control of eye and hand movements used to track visual targets moving in two dimensions. The proposal is based on the premise that studying movements in two or more dimensions reveals unexpected facets of neural control mechanisms. While considerable work has been done on reflexive and saccadic eye movements in three dimensions, multi-dimensional pursuit tracking is less well understood. Recent evidence suggests that there is much more interaction between the brain centers controlling eye and limb motion than previously thought, and this proposal also aims to further the understanding of these interactions in the context of tracking. The proposal has three specific aims. The first aim is to characterize qualitatively and quantitatively the response of the smooth pursuit system to step changes in target direction. Normally, ocular tracking involves saccades as well as smooth pursuit. For this aim, a paradigm has been developed to minimize the probability of intervening saccades. The second aim is to study interactions between ocular and manual tracking of a target. Experiments are proposed in which subjects will track targets moving in two dimensions, with the eye and/or the hand. The hypothesis to be tested is that a gaze signal provides an important input to the limb motor control system and that ocular tracking will be modified to accommodate limb control. Control experiments will test the effects of attention on the ocular response. The third aim is to generalize the results of the first two aims, with the goal of being able to predict normal eye and hand tracking of curvilinear motion. Studies proposed under this aim will determine whether the information used to control tracking is the same as that used for target interception, whether eye and hand tracking is under intermittent control, and whether information about curvature and rate of change of curvature of target motion is utilized in tracking. Control experiments will assess the influence of expectations on the response. The results of these experiments will be incorporated into a quantitative model. The results of this work will characterize normal human tracking behavior. As such they will provide a norm for diagnostic testing in patient populations. Furthermore, this work will identify the logical operations carried out by different brain areas in transforming a visual signal into motor commands to the eyes and to the limbs, and will consequently refine our understanding of brain function.